The most natural and preferable position of a person during sleep is a position in which an upright position is brought down as it is to a face-up lying position. In this state, the back of the head which is the lowest position of the head is placed slightly higher than the back, but the rear side surface of the neck is placed 10 mm to 50 mm higher than the back of the head. Therefore, when the user is taking a face-up lying position, the pillow used as bedclothes should desirably maintain the position of the back of the head to be slightly higher than the back while supporting the neck which is placed higher than the back of the head with appropriate strength.
On the other hand, when the user rolls over during sleep and takes a side lying position, the head will be supported by the shoulder, so the position of the head becomes higher compared to the face-up lying position. Therefore, it is desirable for the pillow to change its height automatically to correspond to the face-up lying position and the side lying position of the user during sleep.
Heretofore, there have been various proposals regarding pillows capable of having its height varied automatically. For example, a pillow is proposed (refer to Japanese Patent Application Laid-Open Publication No. 58-67214) comprising an X-shaped link formed by connecting a link member slanted upward to the right and a link member slanted upward to the left connected via an intermediate pivotable connecting portion enabling the link members to pivot freely and being disposed between a bottom member and a head placement member, with the side on which a crown portion of a head of a user placed on a head placement member is positioned being the rear side and the side on which neck of the user is positioned being the front side, having one end of one of the link members supported rotatably to the bottom member or the head placement member and the other end of the link members engaged slidably in the lateral direction to the head placement member or the bottom member, and further having a tension spring stretched across the upper end of one link member supported rotatably on the head placement member and the upper end of the other link member engaged slidably to the head placement member, so that the X-shaped link is biased toward the upward expanding direction by the tension spring.
There is a description in the above-mentioned publication that when the user places his/her head on the head placement member in a face-up lying position during sleep, the head placement member descends by the load of the head resisting against the biasing force of the tension spring, and on the other hand, when the user rolls over to a side lying position in which the head is supported by the shoulder and the load acting on the head placement member is reduced, the head placement member elevates by the upward biasing force provided to the head placement member by the tension spring via the X-shaped link, and as a result, the pillow can have its height varied automatically according to the position of the user during sleep.
However, there is a drawback in that in order to maintain the head placement member to an elevated position corresponding to the head of the user taking a side lying position resisting against the load of the head, it is necessary to set the upward biasing force at the elevated position of the head placement member to correspond to the load of the head, so that even if the user takes a face-uplying position, the head placement member will not descend responsively, and as a result, load is applied on the neck. Furthermore, there is another drawback according to the prior art pillow mentioned above in that the head placement member is not designed to be depressed by the load of the head, and even when the head placement member is at its most descended position in which the link members of the X-shaped link are folded horizontally, a space corresponding to the width of the link members in the upward/downward direction is formed between the head placement member and the bottom member, so that when the user places his/her head on the head placement member in a face-up lying position, the position of the back of the head is not located at a sufficiently low position. Yet another drawback of the prior art pillow is that since the head placement member is constantly subjected to biasing force in the upward direction, the user must constantly resist against the biasing force in order to maintain the position of the head low, by which load is applied to the neck, causing problems such as stiff shoulders, headache, numbness in the hands and legs, neck pain, sprained neck, and so on.